1. Field of the Invention
The present invention relates to a cleaning unit for removing residue such as left toner employed by electrophotographic systems including a copier, facsimile, printer and an image forming apparatus equipped with the same cleaning unit.
2. Description of the Related Art
According to a conventional electrophotographic system, in toner image forming cycle of charging the surface of a photoreceptor which executes cyclic move, for example, rotation in a predetermined direction about a center axis with a charger, forming a latent electrostatic image on the surface of the photoreceptor by irradiating the surface of the photoreceptor after charged with exposure light and then forming a toner image on the surface of the photoreceptor by developing that latent electrostatic image with a developer, the toner image formed on the surface of the photoreceptor is transferred to a predetermined transfer object surface (recording medium or intermediate transfer body) and finally fixed to the recording medium so that an image composed of the fixed toner image is formed on that recording medium.
Because plural kinds of foreign matters such as not-transferred toner, additive, paper particles and discharge product generated during discharge are left on the surface of the photoreceptor after the transfer to a predetermined transfer object surface is completed, these materials need to be removed with a cleaning means prior to a next toner image forming cycle.
Although various methods have been proposed as the cleaning method for removing the residue such as toner, blade system of bringing a front edge of a plate-like cleaning blade into a pressure contact with the surface of the photoreceptor has been often employed. Also as one of cleaning methods, brush system of using a roll-like brush extending in parallel to the axis of the photoreceptor with bias of opposite polarity to the charging polarity of toner applied to the brush has been proposed.
In the blade system, the surface of the photoreceptor is scraped with a cleaning blade so that the residue on the surface of the photoreceptor is removed from the surface. In such a method using the cleaning blade, rubbing occurs between the surface of the photoreceptor and the front edge of the cleaning blade. Toner particle having a small grain diameter and additive particle added to the toner particle pass through a gap between them thereby substituting for lubricant, so that the rubbing is kept preferable.
By the way, electric and mechanical external forces are applied directly to the surface of the photoreceptor by a charger, developer, transfer means and the like. Thus, the surface of the photoreceptor needs durability against wearing or damage and to secure this durability, a photoreceptor whose surface is hard (see, for example, JP No. 3264218) has been often used.
However, if the photoreceptor whose surface is hard is used, it is difficult to scrape with the cleaning blade. The discharge product generated during discharge adheres to the surface of the photoreceptor and particularly the discharge product becomes hard to remove from the surface of the photoreceptor. Because image flow is induced when the discharge product absorbs humidity, the discharge product needs to be removed completely.
Thus, conventionally, techniques of bringing a fiber body having plural fibers into a contact with the surface of the photoreceptor so as to remove the discharge product with the fiber body have been proposed (see JP-A Nos. 2002-244522, 1-161279 and 5-107993).
Because the blade system is a method of scraping the surface of the photoreceptor mechanically by pressing the front edge of the cleaning blade against the surface of the photoreceptor, if cleaning is executed for a long term, the front edge is worn or damaged or the surface of the photoreceptor suffers from flaws. Particularly, in recent years when toner particles having a small diameter and a shape approximate to a real sphere have been used to meet a demand for high quality picture, the toner particles pass the cleaning blade easily and a pressure of the front edge to the surface of the photoreceptor needs to be increased, thereby accelerating such a problem about the deterioration of the front edge and damage on the surface of the photoreceptor. In this blade system, the toner particles having a small diameter and additive added to the toner particles pass between the surface of the photoreceptor and the front edge of the cleaning blade, thereby substituting for lubricant. Particularly, when a large amount of prints are made and a non-image portion is kept for long hours in a high-speed machine, lubrication is hampered because no toner particle or additive is supplied to the front edge, deterioration of the front edge is accelerated remarkably thereby inducing a drop in cleaning performance.
On the other hand, the brush type has a weaker mechanical scraping performance to the photoreceptor surface than the blade type and consequently, suffers from a small deterioration in cleaning performance with a time passage. The brush type is more advantageous for particularly high-speed machine than the blade type. The brush type makes toner particles attracted by brush fibers (feathers) using electric field positively and further, the brush type is more advantageous in viewpoints of cleaning performance for toner particles having a small diameter and a shape similar to real sphere than the blade type which mechanically scrapes. Because residual toner particles having both polarities exist on the photoreceptor surface after transfer to a predetermined transfer object surface is completed, a technique using two brushes supplied with voltages having different polarities has been proposed. However, another technique has been also proposed which changes polarities of the residual toner particles having both polarities to any one polarity so as to attract the residual toner particles with a single brush supplied with a voltage having an opposite polarity, because the former technique induces enlargement of an apparatus (see, for example JP-A Nos. 63-15278, 2004-239999, 2001-109351 and 2001-3413).
By the way, discharge product generated by charge adheres to the photoreceptor surface and if the discharge product adhering to the photoreceptor surface absorbs moisture, image flow is induced. Thus, the discharge product needs to be removed sufficiently. However, the discharge product cannot be attracted by the brush fibers using electric field different from the toner particles and further, because the brush type provides the weaker mechanical scraping performance as described above, the discharge product cannot be removed sufficiently.
Accordingly, conventionally, a technique has been proposed in which a fiber body having multiple fibers is brought into contact with the photoreceptor surface so as to remove the discharge product by the fiber body (see JP-A No. 2002-244522).
The above mentioned Japanese Patent Application Laid-Open (JP-A) No. 2002-244522 describes a structure which brings the fiber body into contact with the photoreceptor surface in the upstream in the cyclic moving direction of the photoreceptor surface with respect to the cleaning blade by using the cleaning blade and fiber body at the same time. By utilizing the technique disclosed in JP-A No. 2002-244522, it is conceivable to remove the discharge product from the photoreceptor surface by rubbing between the fibers holding the toner particles and the photoreceptor surface while the residual toner particles and additive particles are captured and held by the fibers. However, the residual toner particles and additive particles easy to catch by the fibers have a so small a diameter that the particles having such a small diameter which can substitute for lubricant between the photoreceptor surface and the front edge portion of the cleaning blade are captured by the fibers and consequently, can be short between the photoreceptor surface and front edge portion. If the particles having the small diameter which can substitute for lubricant are short, abrasion and chipping are likely to occur in the blade, so that a favorable cleaning in long periods by the cleaning blade is disabled.
The present invention has been made in view of the above circumstances and provides a cleaning unit in which generation of abrasion and chipping in the cleaning blade is suppressed while adhering substance such as discharge product is removed sufficiently and an image forming apparatus including the cleaning unit.
The techniques disclosed in JP-A Nos. 2002-244522, 1-161279, and 5-107993 have proposed a method of scraping fine residues from the photoreceptor by means of the fiber body and any technique has a such a problem that the scraping performance of the fiber body becomes uneven in the extension direction of the center axis of the photoreceptor. If the fiber body is pressed against the photoreceptor strongly in order to raise the scraping performance by the fiber body, mechanical stress to the photoreceptor is raised and even the photoreceptor having a hard surface suffers from excessive abrasion and flaws thereby accelerating deterioration of the fiber body. Particularly, the techniques disclosed in the JP-A Nos. 2002-244522 and 1-161279 are techniques in which the fiber body is disposed in the downstream in the cyclic moving direction of the photoreceptor surface with respect to the cleaning means and include a fear that the scraping performance by the fiber body is insufficient. Thus, a necessity of pressing the fiber body against the photoreceptor strongly occurs so that the photoreceptor surface is likely to be worn or damaged excessively or the fiber body itself is deteriorated easily. Even if the fine residues are scraped by pressing the fiber body against the photoreceptor surface strongly, the fibers are likely to be saturated with the scraped substances and it is difficult to maintain an excellent scraping performance in long periods.
The present invention has been made in view of the above circumstances and provides a cleaning unit capable of removing fine residues such as discharge product and additives from the photoreceptor surface uniformly and sufficiently in the extension direction of the center axis of the photoreceptor and an image forming apparatus including the cleaning unit.
The technique described in JP-A No. 2002-244522 relates to an technique aiming at scraping the discharge product with the fiber body by rubbing the photoreceptor surface with the fiber body in the upstream in the rotation direction of the photoreceptor with respect to the brush and the residual toner particles adjusted to one polarity by the techniques described in JP-A Nos. 63-15278, 2004-239999, 2001-109351, and 2001-3413 change into a charging condition having an opposite polarity by frictional electrification when they pass the fiber body, so that there is a fear that cleaning of the residual toner particles by the brush may fail.
Accordingly, the present invention has been made in view of the above circumstances and provides a cleaning unit capable of maintaining the cleaning performance for the residual toner particles at a high level while the discharge product is removed sufficiently and an image forming apparatus including the cleaning unit.